1. Field of the Invention
The present invention relates to a piezoelectric material, a piezoelectric device, and a method of producing the piezoelectric device, in particular, a lead-free piezoelectric device whose insulation property is improved by the promotion of crystallization under an excess oxygen atmosphere.
2. Description of the Related Art
A piezoelectric device is typically formed of a lower electrode and an upper electrode, and a bulk-shaped or film-shaped piezoelectric material interposed between those electrodes. As a piezoelectric material, lead-based ceramics such as lead zirconate titanate having a perovskite-type structure (hereinafter, referred to as “PZT”) is generally used.
However, PZT contains lead at an A-site of a perovskite skeleton. Therefore, an effect of the lead component on the environment is considered to be a problem. In order to respond to this problem, a piezoelectric material using a lead-free perovskite-type metal oxide has been proposed.
As a typical lead-free piezoelectric material, there is exemplified BiFeO3 (hereinafter referred to as “BFO”) which is a perovskite-type metal oxide.
For example, Japanese Patent Application Laid-Open No. 2007-287739 discloses a BFO-based material containing lanthanum at its A-site. BFO is a good ferroelectrics, and it has been reported that the amount of remanent polarization of BFO measured at a low temperature is high, and thus is expected to have good properties as a piezoelectric material. However, there is a problem in that a voltage to be applied to BFO for producing piezoelectric distortion cannot be increased owing to low insulation property of BFO under a room-temperature environment.
In view of the foregoing, Japanese Patent Application Laid-Open No. 2007-221066 proposes a method of suppressing an increase in leak current, the method involving substituting iron as the B-site element of BFO with manganese to improve the insulation property. However, BFO whose iron atoms are partially substituted with manganese involves the problem in that a piezoelectric constant reduces.
Japanese Patent Application Laid-Open No. 2005-011931 discloses an approach to substituting the B site of BFO with Co at a ratio of 1 to 10% as an attempt to improving the ferroelectric property of a memory device using BFO. In a conventional method of producing a BFO thin film, however, the amount of undesired phases (hereinafter, referred to as “secondary phase”) which are not of perovskite-type structures increases as substitution with Co progresses. Accordingly, a problem, i.e., a reduction in insulation property arises.
The present invention has been made with a view to cope with such problems, and an object of the present invention is to provide a piezoelectric material whose insulation property is improved while its performance as a piezoelectric material is not impaired.
Another object of the present invention is to provide a piezoelectric device using the piezoelectric material and a method of producing the piezoelectric device.